Current limiting is the practice of imposing an upper limit on the current that may be delivered to a load. The typical purpose of current limiting is to protect the circuit up or downstream from harmful effects due to, for example, a short circuit. In load switch applications used in power sources and adapters, current may be limited below a load switch setting. Load switch applications include driving a power line of a universal serial bus (USB) connector to various peripheral devices. Examples of load switch devices include current limited load switch devices produced by Advanced Analogic Technologies, Inc. (Sunnyvale, Calif.) as integrated circuits (ICs) designed to protect external power ports and to extend battery life in portable electronic products. Such load switch devices operate with an integrated current limiting circuit that protects, for example, the input supply against large changes in load current which could otherwise cause the supply to fall out of regulation.
As current limited devices, load switches are able to draw current up to the load switch setting. If the current exceeds the load switch setting, the current limiting circuit in the load switch limits the current flowing through the load switch. Typically, a resistor (either external or internal to the IC) is used to set the load switch current limits. Typically, within an operating voltage range of the load switch, a single current limit is set based on the resistive value chosen by the designer. One disadvantage of using a single resistor for a wide operating voltage range (and thus a wide range of load current) is loss of accuracy. This loss may occur, because the resistive value and tolerance typically determine the level of granularity of current increments detectable. For example, as shown in FIG. 1, in system design, a user selects a resistor having a resistive value, RSET. The user also defines a current limit to be associated with the resistive value, thereby establishing a one-to-one correspondence between the two parameters. The current limit is arbitrarily selectable by the designer. In this example, the designer has defined the current limit of 100 mA to correspond to a resistive value of 100Ω, and the current limit of 1 A to correspond to 1 kΩ.
A single resistor allows the load switch application to select a single current limit. For example, the same current limit may apply regardless of whether the application is in transmit, receive, or stand-by mode. The current limit detection circuitry may need to be duplicated in order to include in the design capabilities to detect more than one current limit. For example, one current limit may be for stand-by mode and another (e.g., higher) current limit may be for operating mode (e.g., transmit and/or receive mode).
Therefore, there is a need for detectors of multiple current limits. One desired aspect might be to allow for detecting multiple user-defined current limits. Another desired aspect might be to minimize duplication of current limit detection circuitry, if feasible.